It frequently is desirable to monitor the composition of the chemical environment, for example, to regulate chemical or biochemical processes, to determine air or water quality, or to measure parameters of interest in biomedical, agricultural or animal husbandry disciplines.
Because of the nature of the chemical environment, it is desirable that any measurement apparatus have at least some of the properties of: low cost, simple fabrication methodology, digital operation, some degree of signal preconditioning or intelligence, small size, high chemical sensitivity with selectivity, multiple species information with specificity, choice of reversible or integrating response to chemical species, temperature insensitivity or compensation and low power requirement. In addition, the measurement apparatus should have good long term electrochemical stability, good physical resiliency and strength and good resistance to corrosion and chemical attack. In the case of electrical measurement devices, the devices should also have low electrical impedance to provide good signal to noise ratios. With chemically sensitive devices, the devices should also have a Nernstian response to the chemical phenomena being measured.
One method for the detection, measurement and monitoring of the chemical properties of a substance involves the measurement of an electric potential where the potential is dependent upon the chemical activity being measured. Bergveld has proposed that hydrogen and sodium ion activities in an aqueous solution be measured by a metal oxide semiconductor field-effect transistor (MOSFET) modified by removal of the gate metal. P. Bergveld, "Development, Operation, and Application of the Ion-Sensitive Field-Effect Transistor as a Tool for Electrophysiology" IEEE Transactions of Biomedical Engineering, Vol. BME-19, pages 342-351 (September, 1972). In particular, if a MOSFET with no gate metal were placed in an aqueous solution, Bergveld suggested that the silicon dioxide insulation layer would become hydrated and then, because of impurities in the hydrated layer, ion selective. After hydration of the insulation layer of the MOSFET, Bergveld believed the device could be used for ion activity measurement by immersing the device in the solution in question and then recording conductivity changes of the device. Thus, the Bergveld device is commonly referred to as an ion-sensitive field effect transistor (ISFET).
Bergveld's work led to other developments in the field of ion sensitive electrodes such as the chemical sensitive field effect transistor (CHEMFET) device described in U.S. Pat. No. 4,020,830. As described in the '830 patent, the CHEMFET is a MOSFET in which the gate metal has been replaced by a chemically sensitive system that is adapted to interact with certain substances to which the system is exposed. Thus as shown in FIGS. 1 and 2 of the '830 patent, the CHEMFET is identical in structure to a MOSFET except for a sensing layer or membrane 38 that is deposited in place of a metal gate layer on the oxide insulator above the channel region of the transistor and, optionally, an impervious layer 44 that covers all other parts of the CHEMFET that might be exposed to the solution. Numerous variations on CHEMFET structures are disclosed, for example, in U.S. Pat. Nos. 4,180,771, 4,218,298, 4,232,326, 4,238,757, 4,305,802, 4,332,658, 4,354,308, 4,485,274, 4,397,714, and 4,739,380 and in U.S. patent application Ser. No. 07/270,171, now abandoned.
The concept of an ISFET or CHEMFET is especially attractive because of the promise it holds that the high volume, low cost fabrication techniques that are used to manufacture field effect transistors (FETs) in integrated circuits may somehow be adapted for the manufacture of ISFETs and CHEMFETs. Advances in such technology are disclosed, for, example, in the above-referenced U.S. Pat. No. 4,739,380 and U.S. patent application Ser. No. 07/270,171, now abandoned.
One problem encountered in the fabrication of integrated circuits (ICs) is the testing of such devices. Because integrated circuits are so small and yet so complicated, testing imposes major problems in the handling of ICs and in the design of appropriate testing devices and protocols. At the same time, testing is needed as a process control to ensure that the IC manufacturing process is operating as desired and to identify the inevitable number of ICs that do not meet specifications for whatever reason. Testing is a particular problem in the manufacture of ISFETs and CHEMFETs since these devices are transducers which convert environmental variables to an electrical signal. Complete testing of such devices requires that the testing be carried out by exposing the ion sensing or chemical sensing layer of these devices to the environment which the ISFETs and CHEMFETs are designed to measure. One manner of testing is disclosed in U.S. Pat. No. 4,864,229.
A critical component of the testing of integrated circuits is the fluidics head which contains the test fluid which the ISFETs and CHEMFETs are designed to measure. The fluidics head must provide leakproof engagement with the sensing electronic circuit device to be tested so that neighboring devices on the wafer are not contaminated by the test fluid. Additionally, because small volumes or test fluids (on the order of 20-100 .mu.l) need to be handled and because of the small size of the devices, great care must be used to avoid entrapment of air bubbles and fluid contamination during the testing of the devices and upon changing from one test fluid to another. The fluidics head must be designed to test devices lying on a variety of planar wafer materials. These materials include alumina, high melting plastics, glass, silicon, silicon dioxide and silicon nitride.